1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or a copying apparatus, a control method therefor and a storage medium storing a control program. More particularly it relates to an image forming apparatus for providing an image signal with predetermined add-on signals, a control method and a control program therefor.
2. Description of the Related Art
Image forming apparatuses such as printers and copying apparatuses are recently showing remarkable progress in performance, and are becoming capable of forming a high-quality image. In this situation, it is now becoming possible to form images equivalent to bank bills, banknotes and valuable securities, and the problems of forging bank bills or banknotes and infringement on copyrights may increase hereafter.
As a measure for suppressing such problems, for example, a following method is known. It is a signal add-on method, in which, in a color image formed by an image forming apparatus, attached are add-on signals representing tracing information such as a serial number relating to such image forming apparatus, in a state not easily discernible to human eyes (for example cf. Japanese Patent Application Laid-Open No. 2002-202650). This method allows, when a forged bank bill or the like is found, to specify the image forming apparatus that has executed the forging, from the add-on signals attached to the forged bank bill.
Normally, such add-on signals are attached to the entire image. Also, for example, in the case of adding the add-on signals to a color image constituted of yellow, magenta, cyan and black planes, the add-on signals are added only to the yellow plane having low visibility, in order to make the signal less discernible to human eyes.
By attaching the add-on signals to the color image as described above, when an image of which formation is inhibited or a copied image of which copying is inhibited is found, it is possible to extract the add-on signals from such images to reproduce the tracing information, thereby specifying the image forming apparatus that has formed these images.
The add-on signals are attached to all the output images, regardless of whether image formation or copying is inhibited or not. On the other hand, in the image forming apparatus, an intermediate tone process (halftone process) is executed to form screen dots according to the input image data.
In such conventional examples, when the periodicity of the formed screen dots and the attaching periodicity of the add-on signals are in a mutually multiple relationship, the positional relationship between the screen dots and the add-on signals becomes fixed. Therefore, when the screen dots are superposed with the add-on signals, all the add-on signals overlap with the screen dots, whereby the screen dots and the add-on signal become difficult to distinguish and the detection accuracy of the add-on signals is significantly deteriorated. On the other hand, when the screen dot and the add-on signal are adjacent to each other, all the add-on signals are linked with the screen dots and become easily noticeable, whereby the image quality is significantly deteriorated. Particularly a case where the near-by screen dots are linked by the add-on signal, a larger dot is thereby formed whereby the add-on signal becomes more easily noticeable and the image quality is significantly deteriorated.
Also, even when the periodicity of the formed screen dots and the attaching periodicity of the add-on signals are not in a mutually multiple relationship, in the case where the periodicity of the formed screen dots and the attaching periodicity of the add-on signals have a small least common multiple, a proportion of the screen dots and the add-on signals that assume a same positional relationship increases within a given area, thereby leading to the problems described above.